But this seems a bit strange, why should color of pen be Qt::color0? And you need to set color of brush also to Wt::color0, even if you don't want any (Qt::NoBrush). However, drawing a second rectangle (as above) gives:

In my opinion there should be no difference in using QImage::Format_Mono or QImage::Format_MonoLSB when accessing the image just through Qt classes. Only when you operate on the data directly it should be important.